This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A090604 #14 Feb 16 2025 08:32:52
%S A090604 1,2,15,944,725500,9319882896,2537598028922726,
%T A090604 17768840869582166129408,3754397843576564028373337684409,
%U A090604 27480138271604938576005130925123233245100
%N A090604 Number of n-element self-converse groupoids with an identity.
%C A090604 Also partial self-converse groupoids with n-1 elements or self-converse groupoids with an absorbant (zero) element with n elements.
%H A090604 Eric Weisstein's World of Mathematics, <a href="https://mathworld.wolfram.com/Groupoid.html">Groupoid</a>
%H A090604 <a href="/index/Gre#groupoids">Index entries for sequences related to groupoids</a>
%F A090604 a(n) = sum {1*s_1+2*s_2+...=n} (fixA[s_1, s_2, ...]/(1^s_1*s_1!*2^s_2*s2!*...)) where fixA[s_1, s_2, ...] = prod {i>=j>=1} f(i, j, s_i, s_j) where f(i, j, s_i, s_j) = {i=j, odd} (1 + sum {d|i*2} (d*s_d)) ^ ((i*s_i^2-s_i)/2) * (1 + sum {d|i} (d*s_d))^s_i or {i=j == 0 mod 4} (1 + sum {d|i} (d*s_d))^(i*s_i^2) or {i=j == 2 mod 4} (1 + sum {d|i})^(i*s_i^2-s_i) * (1 + sum {d|i/2} (d*s_d))^(2*s_i) or {i != j} (1 + sum {d|lcm(i, j, 2)} (d*s_d))^(2*gcd(i, j)*s_i*s_j/(s_i*s_j*lcm(2*i*j)))
%K A090604 nonn
%O A090604 1,2
%A A090604 _Christian G. Bower_, Dec 05 2003